The main purpose of a hearing aid is normally to amplify received acoustic signals in order to make them audible to the user of the hearing aid. In order to maintain the amplified signals within the user's “comfortable dynamic range”, i.e. the amplitude range between the quietest and the loudest comfortably audible signals, hearing aids typically apply a level compression to the acoustic signals so that louder signals are amplified less than quieter signals. Level compression is particularly useful for hearing-impaired individuals, which typically have a smaller comfortable dynamic range than normal-hearing individuals. In modern digital hearing aids, the level compression is typically achieved in that the hearing aid monitors the level of the received acoustic signals and controls the acoustic gain of the hearing aid in dependence on the signal level. EP1491068B discloses an example of such a hearing aid.
A constant challenge for hearing-aid manufacturers is to help the hearing aid user improve his or her ability to understand speech in noisy environments. A known improvement in this direction is to have the hearing aids preserve spatial hearing cues in the acoustic signals, i.e. information that helps the user in determining the spatial origin of different acoustic signals. Among the most important spatial hearing cues are inter-aural level differences (ILD), i.e. differences in received levels at the two ears for acoustic signals originating from a single source. ILDs are caused by the so-called shadow effect of the user's head and are mainly used for sound source localisation at frequencies above about 1 kHz. The shadow effect causes acoustic signals arriving from the side of the head to be received at a higher level at the ear facing the source than at the respective opposite ear. An individual, who on both ears wears prior art hearing aids like the ones described further above, will, however, perceive reduced ILDs, since louder acoustic signals are amplified less than quieter acoustic signals. This effect of the level compression may reduce the user's ability to determine the spatial origin of acoustic signals and may thus also reduce the user's ability to understand speech in noisy environments.
This is a known problem, and the recent upcoming of binaural hearing aid systems, i.e. hearing aid systems comprising two hearing aid devices, which communicate with each other via a wired or wireless connection, allows binaural processing, i.e. a coordinated audio processing in the two hearing aid devices, and thus allows counteracting the above described reduction of the ILDs. At least one such method is known, which comprises monitoring the acoustic receiving levels at each of the ears as well as increasing the acoustic gain in the hearing aid receiving the louder signal and/or decreasing the acoustic gain in the hearing aid receiving the quieter signal. This method allows for preserving the ILDs—at least in part.